Prochiral ketones, asymmetric hydrosilylation

TABLE 5. Typical results on the asymmetric reduction of prochiral ketones via hydrosilylation... 

Table 1. Typical Results on Asymmetric Reduction of Prochiral Ketones via Hydrosilylation... 

Asymmetric teduction of prochiral ketones via hydrosilylation catalyzed by tfaodium(I) complexes with chiral phosphine ligands... 

ASYMMETRIC REDUCTION OF PROCHIRAL KETONES via HYDROSILYLATION CATALYZED BY L RhCI(S) (6) OR (8)... 

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

The Rh-catalysed asymmetric hydrosilylation of prochiral ketones has been studied with complexes bearing monodentate or heteroatom functionalised NHC ligands. For example, complexes of the type [RhCl(l,5-cod)(NHC)] and [RhL(l,5-cod)(NHC)][SbFg ], 70, where L = isoquinoline, 3,5-lutidine and NHC are the chiral monodentate ligands 71 (Fig. 2.11). 

As outlined in Section II,E, ketone and imine groups are readily hydrogenated via a hydrosilylation-hydrolysis procedure. Use of chiral catalysts with prochiral substrates, for example, R,R2C=0 or R,R2C=N— leads to asymmetric hydrosilylation (284, 285 Chapter 9 in this volume) and hence optically active alcohols [cf. Eq. (41)]. 

Asymmetric hydrosilylation of prochiral ketones continues to be the most popular reaction to examine the efficacy of new chiral ligands or chiral catalyst systems. This asymmetric catalytic process gives enantiomerically enriched secondary alcohols after facile desilylation of the resulting silyl ethers (equation 80). 

Hydrosilylation. Rhodium(I) complexes catalyze the asymmetric hydrosilylation of prochiral ketones (eq 5), in the presence of (—)-sparteine. Secondary alcohols are obtained in up to 30% optical yield by this method. 

Enantioselective Hydrosilylation of C=0 Double Bonds in Ketones. The use of Rh-phosphorane catalyst systems to promote asymmetric hydrosilylation of prochiral ketones with silanes of the type RsSiH has met with only limited success. Thus, hydrosilylation of acetophenone with Ph2SiH2 promoted by [Rh(COD)Cl]2-(S,S)-DIOP catalyst afforded the (S)-(-)-phenylmethylcarbonyl with an optical yield of 32% ee. Similarly, the use of a Rh-NORPHOS catalyst in this reaction proceeded with an optical induction of only 16% ee. S... 

Hydrosilylation of unsaturated organic molecules is an attractive organic reaction. Asymmetric hydrosilylation of prochiral ketones or imines provides effective routes to optically active secondary alcohols or chiral amines (Scheme 756). These asymmetric processes can be catalyzed by titanium derivatives. The ( A ebthi difluoro titanium complex has been synthesized from the corresponding chloro compound.1659 This compound results in a very active system for the highly enantioselective hydrosilylation of acyclic and cyclic imines and asymmetric hydrosilylation reactions of ketones including aromatic ketones.1661,1666,1926-1929 An analogous l,l -binaphth-2,2 -diolato complex catalyzes the enantioselective hydrosilylation of ketones.1... 

Asymmetric reduction of a-keto esters, typically pyruvates and phenylglyoxylates, is effected by chiral rhodium complex-catalyzed hydrosilylation . Optical yields of lactates are higher than those obtained for simple prochiral ketones. The ester group as well as the hydrosilane used effects the extent of asymmetric induction. A high optical yield is attained for M-propyl pyruvate using a-naphthylphenylsilane (85.4% e.e.p ... 

When prochiral silane and ketone are used, hydrosilylation, in the presence of a chiral catalyst, results in asymmetric induction at both the silicon and carbon centers. Treatment of the diastereomeric alkoxysilane by a Grignard reagent leads to recovery of an organosilane and an alcohol of different optical purity. Results obtained in the asymmetric hydrosilylation of ketones and aldehydes by prochiral silanes in the presence of an asymmetric catalyst are summarized in Tables 3 and 4. 

Asymmetric Hydrosilylation of Symmetric Ketones and Aldehydes by Prochiral Dihydrosilanes (68, 79, 80)... 

TABLE 4. Asymmetric hydrosilylation of symmetric ketones and aldehydes by prochiral dihydrosilanes (after References 50, 52 and 53)... 

TABLE 5. Asymmetric hydrosilylation of prochiral ketones by a prochiral dihydrosilane (after Reference 50)... 

Extensive studies on the asymmetric hydrosilylation of prochiral ketones have been conducted by using rhodium(I) complexes with chiral phosphine ligands116 as well as chiral nitrogen ligands194-196 (equation 74). Earlier papers on the asymmetric hydrosilylation of ketones described the use of chiral phosphine-platinum complexes108,134. 

According to the proposed catalytic cycle for the asymmetric hydrosilylation of prochiral ketones1358,147, the a-siloxyalkylrhodium complex plays a key role for the asymmetric induction. The intermediacy of such species is strongly suggested in the hydrosilylation of cyclic terpene ketones catalyzed by RhCl(PPh3)3123, and further supported by spin trapping experiments in which spin adducts such as 24 are detected148,149,128. 

The marked increase in optical yield in the reaction of pyruvates compared with simple prochiral ketones can probably be ascribed to a ligand effect of the ester moiety in the key intermediate or transition state. Further support of this hypothesis comes from the results for asymmetric hydrosilylation of levulinates153 which, followed by acid solvolysis, affords 4-methyl-y-butyrolactone with more than 80% e.e. through the silyl ether of 4-hydroxybutyrates (e.g. equation 83). 

The creation of an asymmetric center by C-H bond formation is a very common process which can involve several types of reactions. Hydrogenation of prochiral olefins is often used with the rhodium catalysts of the Wilkinson type (5). These catalysts were shown to be inactive for ketone or imine reduction except in some cases (15), It was then interesting to develop an alternate method for asymmetric synthesis of chiral alcohols or amines. Since it was found that RhCl(PPh3)3 was able to catalyze silane additions to ketones (16,17) or imines (18), preparation of chiral alcohols or amines by asymmetric hydrosilylation could be envisaged (Figure 2). The 1,4-addition of silanes to conjugated... 

Asymmetric Hydrosilylation of Unsaturated Carbon-Heteroatom Bonds. Asymmetric, catalytic hydrosilylation of prochiral ketones with substituted silanes or siloxanes gives silyl ethers that can be easily hydrolyzed to chiral alcohols. Similarly, prochiral imines undergo asymmetric hydrosilylation to give A-silylamines and, after subsequent hydrolysis, chiral amines (Scheme 32). 

Asymmetric reduction of ketones by hydrosilylation in the presence of a chiral catalyst followed by hydrolysis has been studied by several research groups independently. In this Section, results so far obtained are properly compiled and plausible mechanisms of the asymmetric hydrosilylation of prochiral ketones are discussed. 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

More recently, an alternative mechanism has been proposed [45] for asymmetric hydrosilylation of prochiral ketones using ( f)-DIOP-rhodium(I) complex (6) and a-naphthylpenylsilane, the latter undergoing concomitant conversion into an optically active, bifunctional alkoxysilane, which will be discussed separately (see Section 7.1). According to this proposed mechanism, diastereomeric silylhydrido-rhodium(III) complexes having trigonal bipyramidal structure are assumed as intermediates, which distinguish enantiotopic faces of a prochiral ketone in terms of steric approach control . 

In the preceding Sections it was described that chiral phosphine-rhodium complexes are effective in causing stereoselective addition of a hydrosilane to a variety of prochiral carbonyl compounds to give silyl ethers of the corresponding alkanols with fairly high enantiomeric bias at the carbon atom. The present section describes an application of the catalytic asymmetric hydrosilylation of ketones to the preparation of some new asymmetric bifunctional organosilanes. 

Whereas, for example, the asymmetric hydrosilylation of 2-naphthyl methyl ketone with this catalyst was carried out with 99% yield and 91% ee, the enan-tioselectivities for most aryl alkyl ketones were found to be slightly below those of the most efficient phosphane-based systems. However, the system was found to be exceptionally selective in the hydrosilylation of unsymmetrical dialkyl ketones (Table 15.5), which are difficult substrates [58]. The selectivity for the reduction of prochiral dialkyl ketones was comparable or even superior to the best previously reported for prochiral nonaromatic ketones. 

Cl)2(Tl -diene)2] (diene = COD, NBD) were reported245 to be active catalysts for the asymmetric hydrosilylation of prochiral ketones to give chiral siloxanes, which on hydrolysis afforded the corresponding chiral alcohols... 

In this case, a complex with RhCla gives asymmetric hydrosilylation of prochiral ketones (Scheme 3, Ri = CH3, R2 = CgHs). 

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